Productivity of acacia and eucalypt plantations in Southeast Asia. 1. Bio-physical determinants of production: opportunities and challenges/Productivite des plantations d'acacia et d'eucalyptus en Asie du sud-est. 1. Agents bio-physiques de production determinants: opportunites et defis/Productividad de plantaciones de acacia y eucalipto en el sudeste asiatico. 1. Determinantes bio-fisicos...
Areas of acacia and eucalypt plantations in tropical Southeast (SE) Asia have expanded during the last two decades and there are now at least 2.6 million ha of acacias and 4.3 million ha of eucalypts (Harwood and Nambiar 2014a). These plantations are managed in short rotation cycles typically of 5-8 years, with large areas moving into successive rotations. They are managed to supply wood for the processing industries, mostly pulp and paper production. However, use for eucalypt sawn timber, veneer and composite products (such as medium-density fibreboard) is increasing, for example in China and large volumes of acacia wood are sawn for furniture making in Vietnam. Poles for construction work and firewood are other important uses.
Apart from limited data from experimental studies, there is no consolidated account of management practices and their impacts on the productivity of short-rotation plantations in the region. It is therefore timely to review the available knowledge and management practices of short-rotation forestry as a sustainable land use for wood production. Recognising this, The Australian Centre for Agricultural Research (ACIAR) commissioned CSIRO to undertake such a review (Harwood and Nambiar 2014a), through the following interrelated activities:
* review of the relevant scientific information on the processes that determine sustainability of wood production over successive short rotations, and especially the way inter-rotation management impacts on production
* visits to plantations in the region, guided by local scientists and managers, to view management practices and explore opportunities and challenges
* collection, collation and analysis of wood inventory data from companies to examine trends in production over one or more rotations
* synthesis of this information, identifying major issues and making recommendations for management and future research.
The field work and gathering of productivity data targeted the following selected regions: China (the southern provinces of Guangxi and Guangdong), Indonesia (south and central Sumatra), Malaysia (Sabah), Thailand (central) and Vietnam. To assist readability, they are collectively referred to as SE Asia in the text; noting that not all regions in these countries were covered, and that other SE Asian countries plant acacias and eucalypts. Issues related to afforestation are not explicitly dealt with. Neither are plantations on peat ecosystems, which are largely confined to Indonesia, and face a different set of management challenges. The focus was on acacia and eucalypt plantations grown for wood production on short rotations and on issues related to successive rotations, current and future.
Sustainable production in context
Sustainability of plantation forestry can be judged by the degree of alignment among relevant variables aggregated at an appropriate level (Nambiar 1996; Nambiar and Sands 2013). In the broad sense, the variables determining sustainability include the ecological capability of a site (climate, soil, available water and nutrients), landscape and environmental values, community and social expectations and economic outcomes for investors. When alignment among the variables is strong, risks to sustainability are low, when it is weak, risks are higher. Some of these considerations are outside the scope of this study.
In the specific context of managing acacia and eucalypt plantations for sustainable wood production, the key variables which need to be integrated and examined here are the genetic potential and performance of one or more plantation species, managing threats from disease and pests, impacts of management on site resources and the management intensity required for the desired business outcomes. In this paper, these variables are analysed and their interrelationships discussed in the context of integrated management. Where appropriate, published work from countries such as Australia, Brazil and South Africa, where short rotation forestry is widely practiced, are included. In the companion paper (Harwood and Nambiar 2014b) current patterns of productivity in operational forestry in SE Asia, based on company inventories, are presented.
SPECIES SELECTION AND GENETIC IMPROVEMENT
This section provides a summary of the advances made in species selection and breeding in the region.
Choice of species
The first step towards sustainable production is to ensure that a correct species or inter-specific hybrid varieties that are well suited to the plantation environment and the intended use of wood are chosen. In plantations in SE Asia, eucalypts and acacias are exotics, originating from tropical/subtropical Australia and adjacent Papua New Guinea and eastern Indonesia. Commencing in the 1980s, extensive field testing of many candidate species was undertaken. Interestingly, out of hundreds of species tested, only three acacia and five eucalypt species, together with some hybrid combinations within both genera, now dominate the plantations of SE Asia (Table 1, compiled by the authors from discussions with government agencies, researchers and industry).
Climatic information provides an initial set of criteria for matching species with sites. The acceptable ranges of mean climatic parameters, such as annual rainfall, mean annual temperature and length of the dry season, for good plantation growth are determined from the climates of the natural range of the species, together with those of locations in which the species has grown well in trials or plantations. For example, Eucalyptus dunnii Maiden is suited to subtropical climates and grows well in the highlands of southern China (Jovanovic et al., 2000), while E. pellita F. Muell. is suited to hotter, wetter tropical climates, such as low-elevation sites in Sumatra and southern Vietnam (Harwood 1998). Areas with climates suitable for a species can be mapped using interpolation techniques (Jovanovic et al. 2000). Climatic extremes can cause plantation failures. For example, the tropical acacias do not tolerate frost, or prolonged exposure to cold temperatures just above freezing. Thus, 10 000 hectares of Acacia mangium Willd. and A. crassicarpa A. Cunn. ex Benth, in southern Guangxi province, China, were killed by a cold spell in the winter of 2008. While E. urophylla x E. grandis eucalypt hybrid clones were also damaged, they were more tolerant to cold and were planted to replace acacia. In some regions such as coastal southern China, the risk of typhoon damage also affects choice of species and varieties, and may reduce production (Harwood and Nambiar 2014a).
Species planted beyond their suitable climatic range may be highly prone to diseases. Eucalyptus camaldulensis Dehnh., which in its natural range receives from 250 to 1250 mm/yr (Doran and Turnbull 1997), was planted in the 1990s in central and southern Vietnam, where rainfall commonly exceeds 2000 mm/yr. There, it failed due to attack by leaf blight diseases such as Cylindrocladium quinqueseptatum Boedijn and Reitsma (Booth et al. 2000) and has been replaced by A. mangium and clonal acacia hybrid (A. mangium x A. auriculiformis A. Cunn. ex Benth.). The coastal areas of Binh Thuan and Ninh Thuan provinces in SE Vietnam, which receive 800-1400 mm/yr rainfall, are too dry for these acacia species (Harwood et al., 2007), but suitable for E. camaldulensis to grow well, free of leaf blight (Kien et al. 2008).
Soil type is another criterion to consider in species-site matching. Acacia and eucalypt species planted commonly in SE Asia can grow on a range of soil types; however, there are some notable differences. For example on sandy, seasonally waterlogged soils A. crassicarpa grows faster than do A. auriculiformis and A. mangium, and it can grow well on peat lands. Eucalyptus camaldulensis has greater tolerance of seasonal water logging and soil acidity than most other eucalypt species and is planted on acid sulphate soils in the lower Mekong delta, Vietnam.
Progress in breeding
Intensive testing of species and provenances of acacia and eucalypts has confirmed the suitability and good growth of the widely planted species for their target planting ranges in SE Asia, and also identified superior natural provenances for different sub-regions. Highlights of progress follow.
Provenance selection is an important first step after species selection, as illustrated with A. mangium in Sumatra (Table 2). The fastest-growing natural provenance from Papua New Guinea produced 45% greater volume relative to the local Subanjeriji seed source (commonly planted in the first rotation), and 40% over the southernmost natural provenances from Tully, Queensland (E.B. Hardiyanto, pers. comm. 2013). After suitable provenances of A. mangium were identified (Harwood and Williams 1992), plantations were established using seed from those provenances, and they also provided the genetic base of subsequent breeding programs. There were similar gains from provenance selection for other species.
Breeding programs follow multi-trait improvement objectives that vary from species to species and among organisations. Growth rate has been an important trait in all cases, together with one or more additional traits including stem and branch form, wood properties and disease resistance. Most of these programs started with a genetic base using seed families from more than 100 unrelated parent trees from superior natural provenances. Programs for major acacia and eucalypt species have now advanced to the second generation of breeding (Table 1), and some have entered the third (Luangviriyasaeng et al. 2010).
In eucalypt and acacia, inbreeding (particularly selfing, the extreme of inbreeding) results in loss of vigour of the offspring, regardless of the genetic ranking of the parent tree. Rates of selfing were as high as 50% in natural populations of E. pellita (House and Bell 1996). Reductions in inbreeding contributed to the growth improvement obtained from the first generation of breeding (Brawner et al. 2010). Light or asynchronous flowering of the acacia and eucalypt species discussed here, or seed collection from isolated trees, can result in highly inbred and poorly performing seed, as was shown for A. mangium in Vietnam (Harwood et al. 2004). This points to the importance of good seed collection practices.
In addition to growth, breeding and clonal selection have improved
tree form and wood quality. For example, A. auriculiformis clones now planted in Vietnam have improved stem straightness as well as vigour, relative to previously planted seed sources (Hai et al. 2008). Acacia hybrid clones that are planted in Vietnam were selected from many candidate hybrid genotypes for their superior form, disease resistance and growth (Kha 2001). Some eucalypt clones in China have been ranked for suitability for veneer production as well as pulpwood (Luo et al. 2013).
Evaluating productivity gains from genetic improvement
Differences in growth among genetic treatments in typical breeding trials that test many varieties using small plots are accentuated because of competitive effects, as faster-growing varieties dominate slow-growing ones (Stanger et al. 2011). Results from such trials cannot reliably predict the growth advantage of improved material in operational plantations. Genetic gain trials are required to compare the improved varieties with controls representing those previously deployed, using large plots to minimise competitive interactions. They need to be conducted on the major soil and terrain types, and managed as operational plantations.
Significant increases in growth were achieved by using superior provenances and first-generation improved seedlots, compared to locally available unimproved seed sources. For three genetic gain trials of A. auriculiformis (Hai et al. 2008), conical stem volume at age 4 years averaged 13 [m.sup.3] [ha.sup.-1] for a local seed source, 19 [m.sup.3] [ha.sup.-1] for a mix of the best natural provenances and 28 [m.sup.3] [ha.sup.-1] from select trees in a seed orchard. These seedlots ranked in the same order at three sites in central and northern Vietnam under local management practices. Another trial in northern Vietnam compared the growth of A. mangium seedlots and acacia hybrid. Clones of acacia hybrid grew faster than all A. mangium seed sources. At age 8 years an A. mangium seedlot from selected trees in a seed orchard grew fastest, followed by a seed mix of superior natural provenances and a local commercial seed source. Stem volume of acacia hybrid was 138 [m.sup.3] [ha.sup.-1], 89% greater than 73 [m.sup.3] [ha.sup.-1] for the commercial A. mangium source (N.D. Kien, pers. comm. 2013). These results demonstrate the potential for improving growth through the use of genetically improved planting stock (Table 2).
Genetic parameter estimates from progeny trials of E. pellita (Leksono et al. 2008; Brawner et al. 2010), A. auriculiformis (Luangviriyasaeng and Pinyopusarerk 2003; Hai et al. 2008) and A. mangium (Arnold and Cuevas 2003; Nirsatmanto et al. 2004) suggest that for these species the second generation of breeding will give no more than 510% in volume gain over the first-generation improved material. This is not unexpected, because provenance selection and reduced levels of inbreeding contributed to the large improvements in growth achieved in the first generation.
Clonal versus seed-based deployment
Clonal forestry enables multiplication and planting of selected genotypes to capture both non-additive and additive genetic gain, whereas use of open-pollinated seed from seed orchards deliver only additive gain (White et al. 2007). It has the potential to grow stands with uniform tree size, stem form and wood properties. However, overall plantation uniformity depends on site conditions and management as illustrated by the high spatial variability in growth rates found in clonal plantations of eucalypts and acacia (Harwood and Nambiar 2014b).
Over half of the total plantation area reviewed in this study is clonal (Table 1). The current genetic diversity of clonal plantations of both acacias and eucalypts in SE Asia is low; some growers plant only two or three clones across the entire estate, increasing the risk of severe loss of production to disease or pest attacks.
Pure-species clones of A. auriculiformis (Hai et al. 2008), E. camaldulensis (Kien et al. 2008), E. pellita (Brawner et al. 2010) and E. urophylla S.T. Blake (Kha et al. 2003) have been developed. However, where the environment is well suited to a pure species, breeding programs can provide improved seeds several years before they can deliver thoroughly tested clones. Seeds from seed orchards displayed similar growth rates to selected clones in A. auriculiformis trials in Vietnam (Hai et al. 2008). In South Africa, growers of E. grandis are reverting from clones to improved seed as they provide seedlings with good growth potential at lower cost (Griffin 2014). Some species that are widely planted, including A. crassicarpa, A. mangium and E. dunnii, are not amenable to clonal forestry. So deployment of seed-based planting material will continue in SE Asia. Limited quantities of seed from selected families can be multiplied by clonal propagation of seedlings (without subsequent testing of individual clones). This strategy is now used to mass-propagate selected A. crassicarpa and A. mangium families for large-scale planting in Sumatra, Indonesia (Wong ChinYong, pers. comm. 2013).
Inter-specific hybrids can combine complementary traits of the parents. An example is the E. grandis x urophylla hybrid which combines the disease resistance and higher wood density of E. urophylla and the higher growth potential of E. grandis Hill ex Maiden (Assis 2000). Inter-specific hybrids dominate the eucalypt plantations of China and are important in Thailand, while acacia hybrids are important in Vietnam (Table 1). Hybrids must be planted as clones, not as seeds.
In northern Vietnam, hybrid combinations among E. camaldulensis, E. exserta F. Muell. and E. urophylla grew faster than pure-species crosses involving the same parent trees (Kha et al. 2003). In each of six clone trials in southern China, clones of pure species had markedly inferior growth compared to the fastest-growing inter-specific hybrid clones including the same taxa (R. Arnold, pers. comm., 2013). In a trial at Ba Vi, northern Vietnam, at age 4 years, mean annual height increment was 2.9 m for acacia hybrid clones, compared with 2.2 m for A. mangium seedlings and 1.5 m for A. auriculiformis seedlings (Kha et al. 2012). Acacia hybrid grew faster than the parent species in several other trials in Vietnam (Kha 2001). However, in one trial in southern Vietnam, genetically improved seedlots of A. mangium grew at rates similar to acacia hybrid clones (Kha et al. 2012).
IMPACT OF DISEASES AND PESTS
Some of the diseases and pests of acacia and eucalypts have co-evolved with them in the natural range and move to new areas with the host trees, while some are encountered for the first time in the new environment. Reviews (Dell et al. 2008; Wingfield et al. 2008; Wingfield et al. 2011; Garnas et al. 2012) suggest that pests and diseases pose serious and increasing threats to plantation forests globally as the areas under plantations are increasing. The reasons suggested include:
* Increasing global trade and travel leading to the spread of pests and diseases despite quarantine regulations.
* Plantations are based on a relatively narrow genetic base, and clonal forestry is becoming a norm. The same genetic material may be planted in adjacent countries.
* Species planted to a new environment are exposed to new pests and diseases. For example, guava rust, also known as eucalypt rust (Puccinia psidii Winter), is a disease native to Latin America, which has severely affected eucalypt plantations in Brazil and neighbouring countries (Alfenas et al. 2004) and has appeared in Australia (Kriticos et al. 2013). Similarly, stem wilt/ canker disease, caused by the fungi Ceratocystis acaciivora sp.nov., has spread and is a serious threat to acacia plantations in Sumatra (Wingfield et al. 2011) and Sabah, and has now been identified in Vietnam (Dr Pham Quang Thu, pers. comm., 2013).
As the area of plantations expanded in SE Asia, the potential spread and impacts of diseases and pests were recognised and manuals of damage symptoms caused by them have been published (Old et al. 2000; Old et al. 2003; Thu et al. 2010). Diseases and pests have different modes of action on plants and pathways for spreading. Most insect pests typically consume part of the tree, especially the foliage, whereas a disease affects the physiological functioning of the tree and may kill it. Of course, the two do interact; for example, insect pests may carry disease spores from one tree to another, spreading the disease (Ploetz et al. 2013).
Fungal root rots, predominantly Ganoderma species (Eyles et al. 2008) and stem wilt/canker caused by Ceratocystis (Tarigan et al. 2011) are now causing major loss in production of A. mangium plantations in Sumatra and Sabah. Root rot kills trees in patches and sometimes in large areas. Surveys in second-rotation A. mangium plantations in Sumatra found that from 3 to 29% of trees showed symptoms of root rot (Irianto et al. 2006). In a subsequent survey of 109 compartments in different regions in Indonesia, the percentage of trees with root rot symptoms increased from 5% in the first rotation to 15% and 35% in the second and third rotation, respectively (Mohammed et al. 2012). These data was collected in young stands and, since mortality increases with stand age, losses would be higher by the end of the rotation. Thus mortality due to root rot tends to increase in successive rotations. Because of the impact on productivity, A. mangium is being progressively replaced by eucalypts in Sumatra. A similar change has commenced in Sabah.
Heart rot diseases are a threat particularly to acacia sawlog and veneer log production (Potter et al. 2006) because the recovery and quality of sawn boards or veneer is reduced if the log is decayed. The percentage of logs with some heart rot at harvest ranged from 7% to 47% in five regions of Indonesia (Barry et al. 2004). However, the impact on pulpwood production may be less, because most of the affected trees only had discolouration or incipient decay; less than 10% of the trees had advanced decay or hollows (Barry et al. 2004)). Whether heart rot reduces growth rates of trees is not clear.
In Vietnam, pink disease caused by the fungus Erythricium salmonicolor (Berk. & Br.) Burds. occasionally infests A. mangium and acacia hybrid, killing up to 70% of trees. Infected stands are salvage-logged and the site is replanted to acacia. This is an intermittent threat (Thu et al. 2010), and does not appear to have substantial impacts on productivity.
Fungal diseases that affect the leaves and branches of eucalypts have caused serious damage in some countries. In Thailand, thousands of hectares of clonal plantations were killed by an epidemic of Cryptosporiopsis eucalypti Sankaran & B. Sutton leaf blight fungus in one year (Luangviriyasaeg 2003). In central and southern Vietnam, large areas of E. camaldulensis plantations were lost to attack by leaf blight diseases, notably Cylindrocladium quinqueseptatum (Booth et al. 2000). Another leaf blight, Kirramyces destructans M.J. Wingf. &Crous, has severely reduced productivity in Vietnam and Thailand (Dell et al. 2008). A Botryosphaera canker which attacks stems and twigs seriously reduced growth of a widely planted eucalypt clone in Sumatra (observations by authors). Other diseases, such as the bacterial wilt Ralstonia solanacearum (Yabuuchi et al. 1995) Smith appear sporadically and cause mortality in eucalypt plantations in China (Dell et al. 2008), but so far this has not become a major problem.
An insect of Australian origin, the eucalypt gall wasp Leptocybe invasa Fisher &LaSalle, was first observed attacking eucalypts in the Mediterranean Basin in 2000, and it spread from there to Asia in less than a decade. Gall wasp infests the growing shoots and leaves, stunting new growth and preventing leaf expansion (Mendel et al. 2004). Severe attack will greatly reduce the canopy and wood production. It has severely damaged plantations in many countries (Dittrich-Schroder et al. 2012), including Laos, Vietnam, southern China and Thailand (Dell et al. 2008), where the parasitoids that keep it under control in Australia are not present (Mendel et al. 2004). Different eucalypt clones planted in Thailand vary in their susceptibility to gall wasp (Vitoon Luangviriyasaeng, pers. comm., 2012). This is consistent with the variation among species and clones found in experiments (Dittrich-Schroder et al. 2012). A range of other pests are found in most acacia and eucalypt plantations in SE Asia (Thu et al. 2010). However, other than gall wasp, most do not currently inflict major damage.
Monkeys have become a significant pest of A. mangium plantations in parts of Sumatra. Their populations have increased in their habitat, the protected native vegetation areas within or adjacent to the plantations. They ring-bark the young acacia trees to feed on the sweet-tasting cambium and outer wood. This can kill trees outright, as well as creating wounds which form entry points for Ceratocystis. Elephants also damage acacia plantations in parts of Riau, Sumatra.
WOOD PRODUCTION OVER SUCCESSIVE ROTATIONS: IMPACTS OF MANAGEMENT
In plantations in temperate environments managed on rotations spanning 25-40 years, stands are thinned to harvest logs two or more times, and then clear felled. Thinning is a mild disturbance to site and stand. Thus, these plantations are subjected to major disturbance only once every 25-40 years. In contrast, acacia and eucalypts plantations in SE Asia are currently clear cut every 5-8 years, a major perturbation event, and often intensively managed, exposing the sites to more frequent risks. Even when forests are managed on rotation cycles in decades, management of the inter-rotation phase is critical for sustainability (Nambiar 1996; O'Hehir and Nambiar 2010; Brandtberg and Olsson 2012; Ponder et al. 2012; Wall 2012). In short rotations the options for interventions beyond the establishment phase are very limited. Thus, the management decisions implemented during the inter-rotation phase are critical for subsequent production.
A synthesis of the processes governing sustainable wood production in plantation forests in subtropical and tropical environments and the impacts of management on them (Nambiar and Brown 1997) provides the key principles of management. Selection and deployment of the most suitable genetically improved planting material and its protection from biological threats are essential and these aspects have been reviewed above. Other key interrelated principles for sustainable management follow:
* plantation management operations should ensure that the soil base is protected and disruptions to ecological processes (carbon, nutrients and water cycles) are held within known boundaries of resilience to support longterm productivity, avoiding extreme soil perturbations and conserving site organic matter pools
* due accounting and management of nutrient inputs and exports can inform practices that will enable adequate nutrient-cycling processes within the ecosystem
* the period of inter-rotation management is a window of high risk to the site but also a time of opportunity to correct past mistakes and set the course for sustainable production,
The following sections discuss how the different harvesting and subsequent management practices employed in various regions of SE Asia measure up to those principles and impact on key processes that drive plantation growth.
Methods of harvesting and hauling logs to the log dump or road may be fully mechanical, a combination of mechanical and manual, or simply manual. All these systems are practised in different regions of SE Asia (Harwood and Nambiar 2014a). Their impacts may be directly related to the logging technology and practices, the nature of the terrain, the intensity of biomass removal, and the level of disturbance and damage imposed on soil.
Harvesting practices can influence plantation productivity through soil compaction and loss and displacement of surface organic matter and the nutrients it contains and damage to soil (Powers 2006). However, multi-site studies across the USA and Canada have not found a consistent effect of compaction per se on biomass growth at age 10 years among the common species planted (Powers et al. 2005; Ponder et al. 2012). Harvesting would not compact soil throughout the area; compaction occurs along machine paths and snig tracks. Studies in South Africa have found that the effects of compaction on the growth of eucalypts depend strongly on soils: growth reduction in compacted areas compared with non-compacted area ranging from 2% to 25% depending on soil types. Impacts of deployment of forwarders, loggers, tractors and extraction/snig tracks varied depending on terrain and soils (du Toit et al. 2010). In general, effects of machinery-induced compaction on soil bulk density, strength, porosity, and water-holding capacity have been more demonstrable than effects on long-term stand growth (Ilstedt et al. 2004).
Mechanical harvesting of short-rotation plantations in terrain that is flat or moderately steep can be done with little damage to sites, if slash and litter are retained, operations minimise machine movements across the site and machines traffic over the logging slash. These practices are in operation in A. mangium plantations in Indonesia (Harwood and Nambiar 2014a) where they facilitate subsequent site management for planting.
Biomass displacement and removal
The current post-harvest practices for managing biomass in SE Asia lead to a wide range of impacts (Harwood and Nambiar 2014a). In Indonesia, companies harvesting A. mangium remove only merchantable stem wood, in some cases after debarking at the site, and retain other biomass in situ. At the other extreme, in some regions of China, Thailand and Vietnam all above ground biomass from eucalypt plantations is removed, and in some operations stumps are uprooted and removed as well, for local use as firewood or for bio-energy. Slash burning is not practiced in Indonesia but it is common elsewhere. Biomass export out of the site depends on the harvesting intensity. If slash biomass and litter are burned or displaced (e.g. by windrowing), nutrient losses are unavoidable and nutrient cycles are disrupted. The shorter the rotation, the greater will be the frequency and intensity of export and the degree of perturbation (Folster and Khanna 1997; Mackensen and Folster 1999). The general impacts of such practices are discussed below. In order to follow the fate of above-ground biomass and the nutrients in them during the post-harvest operations, an illustrative summary from a range of sites is given in Table 3.
The amounts of biomass accumulated during stand growth vary according to species, site and rotation length. Management decisions about whether to harvest wood alone or whole trees, and how they are harvested, will impact on nutrient pools. Data in Table 3 show the distribution of biomass and nutrients in stands near harvest age, which allows estimation of export in relation to the intensity of harvest. The amount of wood as a proportion of the total above-ground biomass ranged from 66% to 89% but contained much lower proportions of the nutrients. In contrast, bark mass was 10% or less of above-ground biomass (except for E. tereticornis where it was 28%), but contained much higher proportions of all nutrients. For example, in E. globulus bark accounted for only 10% of the mass but contained 50% of the calcium (Ca), and in two acacia plantations bark accounted for 7% of the mass but 36-39% of the Ca (Table 3).
Whole trees harvest removes the entire live biomass from a site. In other cases slash may be piled at the landing where it may be left, burned, sold off or redistributed back to the plantation. The increases in nutrient removal from whole-tree harvest are two-to-four-fold higher than harvesting wood alone, depending on species, site and nutrient (Table 3). In order to manage the impact of these operations, managers should recognise the amounts of nutrients exported from sites in relation to expected growth rates in the next rotation and the capacity of the soil-species combination in question to cope with such levels of depletion. The progressive increase in the amounts of nitrogen (N) phosphorus (P) and Ca (as examples) removed from sites as a consequence of four intensities of biomass removal from stands of A. mangium in Sumatra and E. grandis in Brazil is illustrated in Figure 1.
Note that the nutrient axes have different scales for the species and nutrients. The patterns of export are similar in both cases but the net amounts and biomass components in which they are accumulated are different. Amounts of N removed from the acacia are far higher than those from the eucalypt. The amounts of P removed for the eucalypt at the Brazilian site are notably higher than those for the acacia in Indonesia. As the amounts and kind of biomass removed increased from merchantable wood only (with debarking at the stump) to whole trees, or the extreme case of complete above-ground biomass including litter, export of N, P and Ca progressively increased (Figure 1) so also for potassium (Nambiar and Kallio 2008) and magnesium (E B Hardiyanto and E K S Nambiar, unpublished). This pattern of nutrient depletion in relation to the harvesting regimes has been demonstrated in other cases, for example, A. mangium in Sumatra (Siregar et al. 2008, S. Siregar pers. comm. 2012) and E. urophylla in China (Xu and Dell 2003). If harvest also included removal of stumps, as is done in some regions, further loss from the site would follow, but there are no estimates of this additional loss for any plantations in SE Asia. Debarking at stump (which is sometimes practised, manually or mechanically) enabling retention and distribution of bark would improve the supply of nutrients (Figure 1 and Table 3).
Site management for the next rotation
Export of biomass and nutrients in merchantable stem wood is inevitable in commercial forestry, but on its own it rarely leads to site degradation. The critical factor is the manager's decision on above-ground biomass utilisation or management and how the site will be prepared for replanting. In regions other than in Sumatra, even when harvesting is largely manual, inter-rotation management often start with practices which include removal of all biomass (including stumps), or burning slash. In some cases, slash and litter are removed by bulldozer to create a clean planting surface (Harwood and Nambiar 2014a), although litter has irreplaceable roles in the healthy functioning of ecosystem processes (O'Connell and Sankaran 1997). These practices do not occur universally in SE Asia, but are widespread enough to warrant discussion and attention in the future.
In subtropical plantations in South Africa, practices such as ripping and sub-soiling gave erratic results--effectiveness, if any, being dependent on soil type, soil water availability and design of the cultivation implements. In some of the terrains in SE Asia (e.g. parts of China, Vietnam and Laos) such operations carried out up and down the slope (rather than along the contour) may do more harm than good. Tree growth in some soils of high bulk density can benefit from ripping, which assists root growth, if carried out with due consideration of the terrain, soil type and the operating season (Goncalves et al. 2013).
Burning slash and litter after harvest, depending on the fuel load, its moisture content and the wind speed, can lead to near complete loss of N in smoke and volatilisation, and partial loss of nutrients including P, Ca and K as particulates. Post-fire ash remaining on the soil surface can be blown away by wind or be washed away with the onset of rains, especially from exposed slopes, as illustrated in Harwood and Nambiar (2014a). Site-specific estimates of potential losses in relation to harvesting intensity can be made from data of the type described in Table 3 to assist management decisions. As an example, in eucalypt plantations, the amounts of N in the slash and litter vulnerable to loss if sites are windrowed or heaped and burned may be between 125 and 500 kg [ha.sup.-1], depending on the biomass and the productive capacity of soils (Fig 1, Nambiar and Kallio 2008). These N losses could be more limiting to production for eucalypts which, unlike acacias, do not fix N symbiotically. Loss of slash and litter will aggravate not only the deficiency of N but also those of P and Ca in some soils. Burning slash and litter may sometimes improve tree growth (compared to their total removal) in the short term (Goncalves et al. 2008; du Toit et al. 2008) because of the ash effect (ash has high concentrations of P and base cations Ca, K and Mg) and higher rates of mineralisation. However, repeated burning would degrade sites in the long term (Goncalves et al. 2007; Goncalves et al. 2008; du Toit et al. 2008; Laclau et al. 2010; O'Hehir and Nambiar 2010).
In plantation soils, the surface 0-5 cm soil horizon has the highest concentration of soil organic carbon and nutrients (see Goncalves et al. 2008; Smith et al. 2008 for examples). It was observed during the field visits that when the heavy blade mounted on a bulldozer pushes forward the slash, it gathers most of the litter and scalps the soil surface. Displacement of even a thin layer of surface soil would have adverse effects on productive capacity of the soil in the long term. Uprooting of stumps is bound to aggravate the problem. If these operations are conducted when the soil is dry, large amounts of fine soil particles ([less than or equal to] I mm fraction) are unearthed, lifted and blown away by wind. If biomass harvest is done in wet weather, soil compaction and erosion are common outcomes. These are damaging practices in tropical plantations (Sim and Nykvist 1991; Panitz and Adzmi 1992; Mackensen et al. 1996; Folster and Khanna 1997; Ilstedt et al. 2004), and pose risks in other ecosystems (Wall 2012; Persson 2013).
Clear cutting and site preparation practices increase mineralisation in soils. When rates of mineralisation are higher than uptake by trees (and associated vegetation at the site), nutrients can be leached if there is water movement. Most of SE Asia receives torrential rains in the wet season. The risk of leaching is moderate to high from harvest and during site preparation and initial growth after planting, but very low or nil once trees reach the sapling stage or close canopy. Risk will be less if the understorey is not completely eliminated, as both growing trees and understorey retained at low density serve as nutrient sinks. If the terrain is steep and exposed to run-off after rains, nutrients and soil may be transported off site with water flow. Removal of slash and litter and inappropriate site cultivation leading to soil and nutrient runoff have been identified as a reason for the site decline in eucalypt plantations in southern China (Xu et al. 2000).
A clear example of the importance of proper management of site organic matter after harvest to the productivity of the next crop of eucalypt hybrids is seen in a study in Congo (Figure 2). Treatments in Figure 2 which resulted in progressive increase from the lowest to the highest volume in a second-rotation stand were: all above-ground biomass including litter removed (R); whole-tree harvest (WTH); wood and bark harvested, slash and litter burned (B); only merchantable wood and bark harvested (TH); wood alone removed, debarked at stump (SWH); slash removed from WTH transported and added on top of the in situ slash = double slash (DS) (Laclau et al. 2010).
When all the slash and litter were removed, stem volume growth declined by 44% in comparison with retention of all but wood on site. Growth increased progressively as the amount of organic matter (and nutrients) retained increased with different management practices. Plots in which the slash and litter were burned grew more wood compared to those in which they were removed, partly because of the nutrients supplied from the ash, but the wood growth was significantly lower than that obtained by retaining the slash. Whole-tree harvesting substantially reduced growth of the subsequent rotation, compared to the best yield at this site, as was also observed for acacia plantations in Sumatra (Hardiyanto and Nambiar 2014).
The results from an international network project including 16 sites in the sub tropics and tropics (Nambiar 2008) found that at a majority of sites there was a reduction in growth rates, in some cases severe, if whole-tree harvesting was practised, even when the litter layer was retained. Conversely, conservation of site resources after wood harvest of the first rotation mostly improved production of the secondrotation eucalypts, acacia, pine and Chinese fir (Nambiar and Kallio 2008). These results have been further confirmed by several experiments in diverse environments and with a range of species (e.g Nambiar and Kallio 2008; Tiarks and Ranger 2008; Tutua et al. 2008; Hardiyanto and Nambiar 2014) and recent reviews (Goncalves et al. 2013; Titshall et al. 2013).
Litter and slash are major sources of nutrients in tropical plantations (Nambiar 2008; Goncalves et al. 2013; Titshall et al. 2013). Their conservation is critical for eucalypts (which unlike acacia species, do not fix atmospheric N because soil organic carbon (SOC) is the primary repository of soil N. Figure 3 shows a strong linear positive correlation between SOC and N in both acacia and eucalypt plantations. There were significant positive correlations between surface SOC and N ([R.sup.2] = 0.89), effective cation exchange capacity (eCEC) ([R.sup.2] = 0.97), exchangeable bases K ([R.sup.2] = 0.81), Ca ([R.sup.2] = 0.50) and Mg ([R.sup.2] = 0.91) in second-rotation subtropical pine plantations (Smith et al. 2008). Furthermore, soil organic matter has a strong influence on key biological, chemical and physical properties including soil strength, porosity and soil water holding capacity.
The benefit of organic matter retention on growth is more than what can be gained by fertiliser application even at sites where SOC concentration levels are relatively modest to high. For example, at an A. mangium site removal of above-ground biomass reduced volume production by 18-20%, compared to retention, but application of P, Ca or K gave no significant growth response to the same stands (Hardiyanto and Wicaksono 2008; Hardiyanto and Nambiar 2014). Similar results have been found with A. auriculiformis in south Vietnam (Huong et al. 2008). These results suggests that slash and litter retention conferred benefits beyond what can be attributed to improved nutrient supply although the exact mechanism is not yet clear.
Site deterioration induced by total biomass removal cannot be compensated simply by adding fertiliser. There are critical questions of efficiency of uptake of nutrients by young trees and losses from the soil which are not understood for the environments in SE Asia. It is not practical to add high rates of fertilisers within a short rotation period without risking off-site impacts, nor is it likely to be feasible or economically sensible under the prevailing conditions of management (Harwood and Nambiar 2014a). Judicious use of fertilisers has a role in sustainable wood production. Many companies are applying fertilizers (Harwood and Nambiar 2014a) but the response to fertilizers in the second and subsequent rotations are uneven (unpublished reports and discussions during field visits 2012-13), partly because of the lack of site specific approaches to fertilizer trials and inadequate understanding of the constraints to production including the availability of soil water. Acacias in Sumatra and Vietnam respond to application of a small dose of P at planting, but a significant volume increase seldom persists to the end of rotation and there has been little or no response to Ca and K in the second rotation (Huong et al. 2008; Hardiyanto and Nambiar 2014). Management of nutrition in the future rotations of eucalypts requires well-designed experimental work.
Repeated ploughing to control vegetation: effectiveness and impacts on trees and soil
Field visits and discussions with managers (Harwood and Nambiar 2014a) identified an important issue: the practice of repeated soil cultivation throughout the rotation and its potential impacts on the ecosystem. In Indonesia, companies who have adopted minimum-tillage practices do not plough plantations. However, elsewhere repeated ploughing to control weeds and to reduce the vegetative fuel load for fire control is common; it is a near-universal practice in block plantations in central Thailand and common in parts of China and Vietnam where the terrain permits. At the current frequency of ploughing (e.g. in Thailand), during the growing of a planted eucalypt crop and the following coppice harvest, together spanning 8-10 years, many sites had inter-rows ploughed up to 20-30 times. At some sites in Thailand and in Vietnam, soil was ploughed when wet, leading to slicing and turning of the clayey soil. These slices turned into hard clods with smooth glazed surfaces when they were exposed and dried. Displaced soils were turned over, creating mounds along the tree rows and 1.0-1.5 m wide depressions, up to 20 cm in depth, between the rows. These became pathways for surface water flow and soil erosion or were prone to waterlogging, depending on topography and soil type. For an illustrated account, see Harwood and Nambiar (2014a).
Is ploughing an effective weed-control practice to improve tree growth and reduce fuel loads?
Ploughing reduces weed growth at best only on about 50-60% of the planted area, because in order to avoid damage to trees the implements cannot approach close to the tree rows. The 1.0-1.5 m wide strips centred on the tree rows remain unweeded. For effective weed control, the spatial location of the weedy and weed-free areas should be the opposite of what is achieved. What is required is an appropriately wide weedfree strip centred along the tree rows, and some vegetation remaining in the inter-rows to minimise soil exposure and erosion.
However, in the absence of any objective appraisal of the biophysical and social aspects of fire risks, repeated ploughing of the entire plantation area continues as the only weed management and fire reduction strategy in many SE Asian landscapes.
Impacts of repeated soil cultivation on root systems
If root growth is not restricted by soil penetration strength, tree root systems in fast-growing plantations will occupy the area between and within tree rows by age one year. This review found no information on root systems in plantations in SE Asia. So a typical distribution of fine roots in soil profiles under E. grandis plantations in Sao Paulo state, Brazil is shown in Figure 4. This study included 16 sites where Site Index (SI measured as mean dominant height) ranged from 22 to 32 m at age 6 years. The three lines in Figure 4 represent the root distribution as means of all 16 sites, the three with the highest SI, and the three with the lowest SI. Root concentrations in the soil both in the surface layer and at depth increased as the SI decreased; that is, the lower the productivity of the stand, the larger the amounts of fine roots produced (Goncalves and de Miranda Mello 2000). In all cases, amounts of fine roots were highest in the top soil layers and decreased exponentially with depth (Figure 4). Despite the two-fold difference in the amounts of roots between site types at 010 cm soil, all values decreased substantially below 20 cm depth. This pattern of distribution characterised by the highest root density in the top soil and a near exponential decrease with depth has also been found in plantations of Pinus radiata D. Don (Nambiar 1990) and eucalypt species grown in a wide range of soils and environments (Bouillet et al. 2002; Falkiner et al. 2006; Grant et al. 2012).
Where repeated ploughing may have induced the formation of an indurated soil layer below about 25 cm the soil surface (Harwood and Nambiar 2014a), it may impede root growth. Subsoil compaction can affect stand growth, through the reduction in root extension to deeper levels from where young plantations take up water during dry months, as was shown for P radiata plantations in South Australia (Nambiar and Sands 1992). Because the fine roots are predominantly in the top soil, regardless of soil type and age of the stands, repeated ploughings will continually mutilate and cut back most of the fine root network in the inter-rows. The effects of this on tree growth may be severe during the dry season, which is a feature of the climates of SE Asia. Potential impacts on coppicing and survival are not known. Furthermore each time roots are cut, trees will have to invest more assimilate for new root growth and this can be at the expense of growth above ground. Repeated ploughing largely eliminates the development of the litter layer, which is important for a number of ecosystem processes.
What are the likely effects of repeated ploughing on soil organic carbon?
There is no experimental work which has examined the impacts of repeated ploughing on soil properties in forestry in SE Asia. Neither is there any information from countries such as Brazil because there zero tillage practices have been the norm for well over a decade. However, there is information from agriculture about the impacts of cultivation and on soil carbon and other soil properties.
What can we learn from crop agronomy? When a forest in Sao Paulo, Brazil was cleared, converted to sugarcane and cultivated regularly, the SOC decreased from 43.4 g/kg to 13.2 g/kg in 22 years and 16.1 g/kg in 60 years, with corresponding declines in labile carbon; reductions in soil carbon pools approaching 63% of their base levels (Lefroy et al. 1995). Some estimates suggest that many soils in the USA may have lost 30-50% of the SOC they held before they were cultivated (Kucharik et al. 2001). A comprehensive review of soil carbon sequestration potential in Australian agriculture concluded that conversion of native land to agriculture involving cultivation and repeated cropping has resulted in decreases in SOC stocks in the order of 40-60% from pre-clearing levels (Sanderman et al. 2009). Among the many studies included in the Sanderman et al. (2010) review, two examples are relevant to this discussion: (i) a chronosequence study which showed that when land was continually cultivated, SOC dropped from a base level of 22 g/kg to 15 g/kg in 10 years and to 6 g/kg by 45 years, a 73% reduction and (ii) applying a single tillage to a plot which was not tilled (NT) for the previous 14 years resulted in large losses of SOC from the soil but applying NT to a previously tilled plot resulted in no significant gain in SOC. Soil cultivation (tillage) disrupts soil aggregate structure which would increase decomposition rates of carbon pools but the re-formation of stable aggregates which protects carbon pools is a much slower process even after tillage is completely ceased (Balesdent et al. 2000). Field observations at the plantation sites which were repeatedly ploughed confirmed that at some sites soils have lost all structural elements including aggregates to a depth of 25 cm (Harwood and Nambiar 2014a).
A number of studies in agricultural systems show that the rates of decline in SOC in response to soil management is faster than rates of recovery from reduced levels (Sanderman et al. 2010). Based on modelling studies on Pinus radiata and E. globulus plantations grown in Australia, the predicted general rate of decrease in SOC during the first ten years after planting was 0.79 t C ha/yr (1.7% per year) and the predicted rate of increase from age 10 to 40 years was 0.46 t C ha/yr (0.82% per year), much slower than the rate of decrease (Paul et al. 2003)). Many of the sites contributing to the database for the studies of Paul et al. (2002; 2003) studies were disturbed, sometimes intensively, before afforestation or reforestation. The discussion above shows that repeated ploughing in short (5-8 year) rotations may be leading to declining soil organic matter levels in the soil, which would likely have a negative effect on productivity. However, as discussed below, if aboveground biomass (other than stems) is retained at sites and the site is not cultivated for replanting, SOC levels can be maintained or slightly improved (Table 5).
Soil-available water and plantation growth
Tree species have specific climatic requirements, especially in their physiological ability to cope with stresses such as water deficits. Even when soils low in chemical fertility can be supplemented to a certain extent with nutrient additions in fertilisers to improve production, a growth response can occur only if there is adequate soil-available water and if there are no constraints in soil physical properties such as compaction that impair root growth. Soil-available water is often an overriding factor determining variation in the productivity of plantation forests across local landscapes. For example, in E. globulus Labill. plantations in Western Australia nearly all the variation in volume growth rates across sites could be explained by a combination of climate wetness index and soil depth (White et al. 2009). Similarly, two key variables--atmospheric vapour pressure deficits and soil water availability --accounted for most of the variation in growth across a large area of eucalypt plantations in eastern Brazil (Almeida et al. 2010). Further work from Brazil (Stape et al. 2010) showed that soil-available water can be an overriding site variable determining growth rates in areas with seasonal water deficit, a common feature in plantations areas in SE Asia. For example, at a site in South Vietnam with an annual rainfall of 2500 mm/yr, stem diameter growth of A. auriculiformis between 3 and 4 years of age ranged between 2-3 mm/month during the rainy season but declined to 0.5 mm/month or zero during the dry months from December to March (Huong et al. 2008).
Total annual rainfall is not a reliable measure to explain growth rates or to understand the scope for improving production with management inputs, because the net water available to the stand throughout the rotation depends on factors including frequency and seasonality of rain, evapotranspiration, soil properties determining water-storage, and stand management. Some options for managing available soil water and improving production include: conserving organic matter, especially on moderate to steep slopes; managing soil in ways that do not cause erosion or degrade soil structure; and, under some circumstances, improving water infiltration in soil and rooting by deep ripping. Controlling competing weeds is an assured way to increase the share of soil water and nutrients available to planted trees. Studies on the relationship between site factors controlling available soil water and tree growth is a critical area of research in SE Asia.
MAINTAINING PRODUCTIVITY AND SOIL PROPERTIES OVER SUCCESSIVE SHORT ROTATIONS: EXPERIMENTAL EVIDENCE
In the previous sections, the impacts of some of the common site management practices deployed for harvesting and replanting on productivity of successive rotations have been discussed. Recognition of the importance of building knowledge of the biophysical processes governing the productivity of tropical plantations (Nambiar and Brown 1997) and the rate of expansion of plantation forestry in the sub-tropics and tropics prompted the development of a coordinated network project under the aegis of Centre for International Forestry Research (CIFOR 1997 to 2008) in partnership with local public institutions, universities and companies in Australia, Brazil, China, Congo, India, Indonesia, South Africa and Vietnam with supporting sites in Southern USA. The core aims were to examine questions about the productivity of successive rotations and to support capacity building among some partners. The project included 16 sites, 14 in the subtropics and tropics and two in Mediterranean environments (see Nambiar and Kallio 2008 for details). A unique feature of the study was that a set of core treatments was implemented uniformly across all sites with optional treatments appropriate for the local situations. Results from this study have been cited in some contexts earlier. Although the project was formally completed in 2008 and final proceedings were published (Nambiar and Kallio 2008), work has continued at several sites. Using more recent and updated results (through personal communications by the authors) and recent published research from other sources, three key questions relevant to sustainability can be addressed:
* Can productivity of tropical plantations be maintained at sites for more than one rotation?
* What is the scope for increasing production with management and across key species?
* What types of changes might there be in soil properties?
Can productivity be maintained?
Questions about productivity across multiple rotations in the tropics cannot be answered in detail, because the history of tropical plantations is recent and relevant data are not easy to find (see (Harwood and Nambiar 2014a). Such comparisons are scant, even for temperate plantation forestry systems, despite their long history. An example for P. radiata is provided by O'Hehir and Nambiar (2010). The CIFOR network project examined the productivity of first-rotation (1R) and second-rotation (2R) stands in commercial management units across ten eucalypt, four acacia, one hybrid pine and one Chinese fir site. Earlier results (Nambiar and Kallio 2008) showed that 11 out of 16 sites grew at faster rates than in the corresponding previous crop, some substantially so. When earlier results were summarised (Nambiar and Kallio 2008), none of the four acacia sites were at the end of the second rotation. Full rotation results for 1R and 2R for all eucalypt and acacia sites are now available, and are presented in Figure 5. The genetics of planting stock changed from 1R to 2R which also received, in general, better weed control than in 1R. The MAIs for 2R are the mean of several experimental treatments, so they represent the mean effect of a range of management practices at each site. The dotted line in Figure 5 shows the 1:1 parity. Second-rotation yields for the majority of sites were higher (in some cases nearly three times higher) than the first, and others were close to the 1:1 line with no case where there was a clear decline in production.
There are several limitations in interpreting the differences in growth rates and net production between rotations (O'Hehir and Nambiar 2010). For example, growth rates are influenced by factors including the planting of different genetic material, climate, stocking, pests and diseases and management and these cannot be held constant across rotations. However, if genetically improved planting stock is used and management practices which conserve site resources are applied, productivity can be maintained and enhanced. For example, in experimental studies of A. mangium in Sumatra, MAI increased from 29.4 [m.sup.3]/ha/yr (1R) to 43.0 [m.sup.3]/ha/yr (2R) at one site and from 29.7 to 47.8 [m.sup.3]/ha/yr at a second site (Hardiyanto and Nambiar 2014). Similarly, an experimental study on A. auriculiformis in south Vietnam demonstrated the progressive improvement in MAI from 11.0 [m.sup.3]/ha/yr (1R) to 28.3 (2R) and 33.0 [m.sup.3]/ha/yr (3R) (Vu Dinh Huong et al. 2014 unpublished report, Vietnamese Academy of Forest Science). The potential for increasing production and the net benefit of improved management on production are high. Such studies are much needed on eucalypt plantations in SE Asia.
What are the prospects for increasing production within a rotation?
Table 4 summarises the changes in wood production in response to management inputs at ten eucalypt and three acacia sites at the end of the second rotation across a range of environments and growth rates. Results show opportunities for improvements at most sites. The lowest and the highest volumes were obtained from the range of treatments (planting stock remained the same within each site); the lowest was usually associated with depletion of organic matter and the highest with the retention of organic matter and supply of additional nutrients in some cases. The main points are that the prospects of increasing production with better site management vary but are often high, and that opportunity for increasing production applies to both low- and high-quality sites and across a range of species and growing environments.
What are the trends in changes in soil properties?
A common concern about forestry in the tropics has been the consequences for the soil of the removal of biomass and nutrients by harvests at intervals of 5-8 years. Several reviews have summarised the potential depletion of nutrients, especially base cations (K, Mg and Ca), and raised the prospect of increasing soil acidity (Folster and Khanna 1997; Nykvist 2000; Mackensen et al. 2003).
Based on earlier results, Tiarks and Ranger (2008) concluded that, across all sites (Table 4), changes over the rotations in soil organic matter (SOC), N, pH and exchangeable cations were small and transient and there was no indication of a decline in surface soil properties, unless extreme treatments were applied such as total removal of above-ground biomass. Using their summary and updated results obtained through personal communication with partners, trends in the direction of the changes (across core treatments) for six eucalypt and three acacia sites for a full rotation period are summarised in Table 5.
In general, changes in soil properties were small and none were consistent across sites. Soil pH remained stable over the rotation length. There were indications of reduction in exchangeable K in four out of nine sites and a trend in decline in extractable P for the acacia sites. Soil organic carbon levels remained relatively unchanged or increased at four sites (Table 5). At an E. grandis site in Brazil, Goncalves et al. (2008) followed SOC annually at 0-5 cm and 5-10 cm depths from planting to the end of the rotation at age 7 years. The site was replanted under minimum tillage according to local operational practice. In both the soil layers, SOC concentration remained stable (close to 20 g/kg in 0-5 cm and 10 g/kg in the 5-10 cm) from clear-cutting the first rotation to the end of the second rotation. This result is consistent with that found in a study in a E. globulus plantation in Western Australia (Mendham et al. 2008) and from a range of other sites (Table 5).
Concerns about soil acidification (declining pH) in acacia plantations have been raised, although seldom substantiated by reliable data. The suggested reasons are these species can grow fast, enabling uptake of large amounts of mineral nutrients in the biomass which may be removed at harvest, and they fix atmospheric N which in turn may increase N leaching and carry bases in the leachate. Because this concern is recurring in SE Asia (Yamashita et al. 2008; Sang et al. 2013; Dong et al. 2014) a discussion is appropriate. Yamashita et al. (2008) compared soil pH under stands of first-rotation A. mangium with those from secondary forests and Imperata grasslands in south Sumatra. They found no significant difference in soil pHH2O up to 30 cm depth between secondary forests and A. mangium plantations. In fact, pHH2O in A. mangium was slightly higher than in secondary forests, and pHKCl under A. mangium was generally higher than in both secondary forests and Imperata grasslands. The difference in pH between grassland and acacia may be due to a pH rise under grass (possibly an ash effect arising from frequent burning by farmers) and not a real decline under acacia, where pH was slightly higher than under secondary native forests. Distribution of exchangeable bases in the profile showed similar patterns to those of pH and bases in soils under native forest and acacia were identical. Thus, these results do not support the conclusion by Yamashita et al. (2008) that acacia plantations per se reduced soil pH in one rotation.
Dong et al. (2014) compared fallow shrub lands in central Vietnam below overhead powerlines, with a chronosequence of adjacent acacia hybrid plantations from age 0.5 years to 5 years. To start with, fallow land had soil properties very different from the land under the acacia series. For example, total SOC in the upper 20 cm of soil in the fallow land was 12.99 [+ or -] 1.75 (SE) Mg/ha and that in the plots of 0.5 year old acacia was 21.76 [+ or -] 0.72 (SE) Mg/ha, some 40% higher. This large difference in SOC cannot be attributed to 0.5 years of growth of an acacia stand. Despite such serious confounding, and based on a difference of 0.1-0.2 units of pH, it was concluded that soil acidity increased under acacia plantations. In another matched-plot study in Vietnam, Sang et al. (2013) found that the mean [pH.sub.H2O] across several sites were: plantations 4.4, secondary forests 4.5 and pastures 4.9. The pH difference between plantations and secondary forests was not statistically different, but that between plantations and pastures was. This small difference, with pastures higher by 0.6 pH units, could have been due to practices common in local farms (e.g burning) and is not a reliable measure of decline in pH under plantations or native forests. The cation exchange capacities for plantations, secondary forests and pastures did not differ significantly. Yet, it was concluded that 'all plantations, but not secondary forests, caused increases in soil acidity'.
The pH measured annually during the full second-rotation period for three acacia plantations at two sites in Sumatra (Site 1 is a red yellow podzol and Site 2 is a Ferric Acrisol) and one site in southern Vietnam (Chromic Acrisol) is presented in Figure 6. Data are from plots where stem wood (with bark) was harvested, and slash and litter were retained, as is the common practice in Indonesia. The first data point is the measurement at the end of the first rotation. Soil pH fluctuated through the years, in some sites more than in others, but there was no indication of pH changing with stand age during the full rotation period. Similar results were found at two E. globulus sites in contrasting soils in Western Australia measured over ten years (Mendham et al. 2008).
Studies using chronosequences and paired sites that have concluded that acacia plantations acidify soil overlooked the serious confounding effect of differences between land types and their history and that conclusions drawn from one-time measurements can be misleading, as the pH fluctuates in time (Figure 6). The idea that acacia plantations acidify soils appears to be widespread, yet measurements over a rotation from a set of designed experiments (Figure 6, Table 5) do not support that conclusion. It is important to build a long-term database from appropriate experiments to monitor and understand the impacts of management on soils over several rotations, and to use such information to improve management and to address questions from communities.
WAYS FORWARD FOR PLANTATION RESEARCH AND MANAGEMENT
Sustainable wood production with economic viability and environmental care provide the foundation for successful plantation forestry. For achieving this goal, managers in temperate zones, for example, Australia, New Zealand and SE USA have been backed by more than a century of experience and substantial investments in R&D. Elsewhere, short rotation forestry with eucalypts and pines had the support of high quality research, technology adoption mechanisms and decades of experience, as illustrated in South Africa (Morris 2008; Titshall et al. 2013) and Brazil (Alfenas et al. 2004; Stape et al. 2004; Goncalves et al. 2013). In comparison, large scale development of eucalypt and acacia forestry in SE Asia is recent with some estates less than 10 years old, and informed by modest and fragmented research (Harwood and Nambiar 2014), some of it not easily transferable between countries as it is reported in different languages.
Because the history of forestry of SE Asian countries and regions within them, ownership of the land base, the species grown, investments in management, the degree of vertical integration and cultural backgrounds all differ, an analysis does not yield conclusions uniformly applicable to all. For example, a strategy for dealing with serious threat of diseases of A. mangium including change of species, bio-control and genetic selection is among the highest priorities for Sumatra and Sabah. In comparison, pests and diseases of eucalypts in China or Thailand, while warranting close surveillance, are not currently an over-riding threat, whereas the need for major improvements in site management for improving productivity is clear (Harwood and Nambiar 2104a). Pre-emptive strategy to prevent the spread of diseases and substantial efforts to avoid site-damaging practices are high priorities for Vietnam. Special challenges for adoption of improved practices arise from that country's highly dispersed pattern of plantation ownership, over 40% of plantations being managed by smallholder farmers.
The preceding sections have identified issues which, while not universally holding across all regions in SE Asia, are important enough to warrant analysis based on available information.
Future contributions from breeding
The last two decades of selection and breeding has laid a strong foundation to support decisions made on the choice of species suitable for the goals of forestry in SE Asia and there has been notable progress in developing improved genotypes which have been taken up for operations in small and large companies (Table 1). Three key issues for the future are:
* The need to provide stronger evidence of the value added by deployment of genetically improved planting to stock to production at operational scale
* Constraints on the quality and supply of improved materials to all growers
* The need to re-prioritise breeding objectives
In experiments, genetically improved planting stock has consistently given higher growth than unimproved material. What is not clear is its contribution to increased production in operational forestry, partly because few results from genetic gain trials are available for acacias and eucalypts in SE Asia. It is necessary to establish ongoing evaluation of improved genetic material covering the spatial variability and management across the estate to benchmark progress in breeding and justify ongoing investments in breeding.
While virtually all eucalypt planting stock in some countries is now clonal (Table 1), in parts of SE Asia (e.g. for A. mangium and E. urophylla in Vietnam), a major proportion of the planting materials are still raised from seeds collected from natural provenances or from unimproved local plantations. This is particularly the case for smallholder growers, and efforts to provide them with genetically improved seed or well-tested clones should continue.
Diseases and pests pose severe risks to production in SE Asia. This calls for a revision of breeding objectives. Future breeding should give high weighting to traits that reduce risks to sustainable production, rather than primarily targeting further major increase in growth potential, which is in any case unlikely to be achieved now that improved varieties from the first one or two generations of breeding are widely available. Breeding should of course also continue to improve value and profitability, through improvement to traits such as tree form and wood quality.
Resistance to diseases and pests can be improved by (i) selection within species or (ii) inter-specific hybridisation. Changing to a more resistant species is another option.
Eucalyptus pellita is resistant to a range of tropical diseases (Harwood et al., 1997; Harwood, 2008, Guimaraes 2013). It can be hybridised with less-resistant species such as E. camaldulensis, E. dunnii, E. grandis, E. tereticornis Smith and E. urophylla. This offers the prospect of incorporating resistance to disease in multi-species hybrid combinations, along with desirable characteristics from the other parent species. This approach has been implemented in Brazil (Resende and Assis, 2008) and should be considered in SE Asia. Differences in susceptibility to the eucalypt gall wasp among eucalypt species, provenances, inter-specific hybrids and individual clones (Pham et al. 2009; Chang et al. 2012; Dittrich-Schroder et al. 2012; Kriticos et al. 2013; Luo et al. 2013) can also be exploited through breeding.
Acacia mangium and A. crassicarpa are highly susceptible to Ganoderma root rot and Ceratocystis stem wilt/canker. Initial indications are that there appears to be little genetic variation within A. mangium in susceptibility to root rot (Eyles et al. 2008) or Ceratocystis (J. Brawner, pers. comm. 2013). No acacia species with superior disease resistance appear to be available for planting or hybridisation with A. mangium.
In Brazil, before large-scale release, eucalypt clones are systematically screened for resistance to major diseases by inoculating young trees with the diseases and monitoring their tolerance to infection under controlled conditions and in the field (Dehon et al. 2013). Systematic screening is strongly recommended for SE Asia, given the increasing problems from serious diseases such as eucalypt rust (Puccinia psidii) which are spreading globally (Kriticos et al. 2013) and can be expected to reach SE Asian countries.
Pathogens can evolve rapidly, attacking previously resistant eucalypt genotypes (Graca et al., 2011). Thus it is essential to maintain ongoing selection and breeding, based on diverse populations of the parental species. Unless supported by an ongoing breeding and clonal testing program, clonal plantations represent a genetic dead-end, with no capacity to adapt. Ongoing pure-species breeding and maintenance of a wide genetic base are also needed for development of new inter-specific hybrid varieties. It is important that SE Asian breeding programs retain the genetic diversity of the original base populations, to access genetic variability to counter emerging risks or meet new market requirements.
Clones developed in one region should not be planted in commercial scale outside that environment without rigorous testing in the new region. An example illustrates the high risk: clones of the E. urophylla x E. grandis hybrid, developed in subtropical southern China, have repeatedly failed in the equatorial environments of Sumatra and Borneo, because of their high susceptibility to fungal leaf blight diseases.
Managing impacts of diseases and pests
Acacia and eucalypt plantations straddle the land borders between several SE Asian countries. Large volumes of logs are traded within the region (e.g. A. mangium logs with bark are transported from Malaysia to Vietnam) and these could carry fungal diseases and insect pests. The eucalypt gall wasp has spread from country to country within the last decade. In view of the major economic importance of plantations in the region and the current serious disease threat to acacia plantations there is a clear need for governments and the private forestry sector to develop coherent surveillance programs within and across national boundaries. Within a country or local region, quarantine regulations can also reduce the rate of spread. International quarantine regulations play key roles in containing or slowing the spread of diseases and pests around the world (Garnas et al. 2012).
Investments in R&D in biological control of root rots using Trichoderma fungi may offer a partial solution to root rot (Eyles et al. 2008; Mohammed et al. 2012). Similarly introduction of Australian insect parasitoids of the eucalypt gall wasp has been successful in Israel (Kim et al., 2008). Parasitoids indigenous to the environment where gall wasp outbreaks occur may also eventually control gall wasp to low levels, although biological control should be integrated with tree breeding to improve resistance to wasp attack.
Sustaining production over successive rotations
The productivity of a plantation site is not an immutable reference point; it is a snapshot in time because site productive capacity can be upgraded by good management or downgraded by poor management (Nambiar 1999). Similarly, changes in species or varieties in successive rotations can influence productivity, so too incidence of diseases, pests and changes in climate. Results from studies on long-term productivity of plantation forests trace back trends in production to factors including the choice of species and varieties (Turnbull 2007), soil- site management practices and overall quality of the management adopted by the enterprise (Nambiar 1999; Powers 1999; Morris 2008; O'Hehir and Nambiar 2010; Nambiar and Sands 2013). Some argue that short-rotation forestry involving large-scale planting with a single species will inherently face high risks. There is no evidence to back this assumption as a global case (Powers 1999; Nambiar and Sands 2013). There are several successful plantation ventures in temperate regions with one species (e.g. P. radiata plantations in Australia, New Zealand and Chile), and with short-rotation eucalypt plantations over several decades in tropical and subtropical countries such as Brazil and South Africa. However, the recent experience with disease outbreaks in A. mangium plantations in Indonesia suggests that as well as breeding and management to address risks to widely planted species, there is an ongoing need to develop alternative plantation species in case species changes are necessary.
The productivity trends from operational inventory (Harwood and Nambiar 2014b) indicate significant risks and constraints to achieving the goal of sustainable production in SE Asia, unless cohesive and coordinated efforts to address the constraints to production are put in place. Expectations of ongoing high rates of growth and productivity cannot be met if management practices damage ecosystem properties and processes in ways that are hard to reverse. The loss of productive capacity of soils through practices which deplete soil organic matter and cause losses of surface soil in parts of SE Asia is detailed in field reports (Harwood and Nambiar, 2014a) and discussed earlier in this paper.
Multi-site experiments in subtropical and tropical plantation sites with key species provide evidence to explore three issues critical for successive rotations. If management practices which conserve site resources are employed, productivity can be maintained and often improved over successive rotations (Figure 5). Within a rotation there is much scope for improving productivity of both eucalypts and acacia (Table 4, Figure 2). Overall results and the review of published information show that key properties of soil can be maintained across rotations (Table 5, Figure 6) if proper site management practices are followed. These important findings are supported by long term growth studies in temperate forestry (O'Hehir and Nambiar 2010) and by research in short rotation eucalypts forestry in South Africa (Titshall et al. 2013) and Brazil (Goncalves et al. 2013).
The major challenges in the context of SE Asia have multiple dimensions that require integrated rather than fragmented action. These include:
(i) avoiding practices which, by all known accounts, impose high risks of long-lasting soil degradation;
(ii) developing applied research on key issues; and
(iii) integrating inputs from genetics, crop protection and system management into locally specific packages of practices and achieving their operational application.
Avoiding damaging practices: There is an adequate body of information and experience to conclude that bulldozing, removal of total biomass at harvest, including stumps, burning of slash and litter and intensive site disturbance especially on steep land with shallow soils are operations that pose high risks to processes that govern sustained productivity. Evidence is strong that depletion of site organic matter and associated nutrients will result in loss of production in subsequent rotations under a wide range of conditions. It is also suggested that, in general, forestry in SE Asia will be better served in the long term by further developing and adopting management practices that promote conservation of site resources, augmented by site-, species- and problem-specific use of fertilisers, based on evidence. It is essential to recognise that investments in advanced breeding will be of no avail if site resources are depleted and soils damaged during harvest and inter-rotational management. Therefore, it is important to act on this now, using the best information available, while setting in place applied research which will also serve to demonstrate the impacts of site management practices and generate long-term data to quantify these effects.
What are the key research questions? These have been discussed in detail elsewhere (Harwood and Nambiar, 2014a) and only key issues are highlighted here.
Inventory data from Sumatra suggest that when eucalypts are planted as a replacement for A. mangium, the growth rate of eucalypts is lower than that obtained from A. mangium before encountering the disease problems (Harwood and Nambiar, 2014a). Studies on constraints on the productivity of eucalypt in humid tropics would be of much benefit.
Commercial plantation forestry has not been possible, anywhere, without proper methods for managing vegetation. Currently, use of herbicide technology is the most common practice in Indonesia. Companies are committed to repeated applications and a zero weed tolerance policy. However, contemporary approaches in vegetation management do not aim for zero tolerance of all vegetation other than the commercial trees; they aim to achieve balance between production goals and acceptable limits to vegetation. Under such management, some weeds can serve as a source of organic matter and contribute to nutrient cycling.
In other regions ploughing remains very common both for weed control for improving tree growth and also as the sole fuel reduction strategy, giving rise to repeated ploughing every year as the sole fire reduction activity. While there is no local research on the comparative merits and demerits of this practice, evidence from agriculture is strong that repeated soil cultivation leads to loss of soil organic matter and soil structure. The field observations reported by Harwood and Nambiar (2014a) suggest that integrated research aimed at (i) systematic analysis of the cause and spread of fire, (ii) testing the effectiveness of repeated ploughing on plantation growth including the long term impacts on soils and (iii) exploring the judicious adoption of herbicide technology is among the high priority areas for some regions in SE Asia.
There is very little knowledge on the factors determining soil available water and their likely strong influence on the eco-physiology of productivity in most plantation areas in SE Asia. Large regions within SE Asia have pronounced dry seasons, and site management practices may have significant effects on soil water storage. Silvicultural inputs such as fertilizer application and weed control are likely to be more effective if they are implemented with knowledge of the spatial and temporal variations in soil available water.
Integrated management is the way forward
The critical step required for future success is to develop an integrated approach to plantation system management that takes into account, simultaneously, the major determinants reviewed in this paper, rather than focussing on one at the expense of others. In countries such as Vietnam where a significant part of plantation forestry is in the hands of many thousands of small growers and farmers, assisting them to adopt this approach will be no small task.
Several examples illustrate the necessity for an integrated approach. Given the strong evidence that inter-rotation management is a critical phase of risks and opportunity for the sustainable management of short rotation forestry, harvesting practices should make subsequent management for planting more efficient, biologically and economically. It is neither biologically sensible nor practical to expect that loss of total biomass and the nutrients in it by whole tree harvest with the non-wood components going to bio-energy production can be compensated by addition of fertilizers, without economic and environmental costs. Many managers in SE Asia view genetic improvement as a panacea for securing improved productivity. However, no matter how much is invested in breeding, clonal plantations cannot provide uniform and high growth rates unless the clones have been rigorously tested across the range of site types in the target landscape and in relation to major pests and diseases, and plantations have been managed so as to maintain and where possible enhance site quality.
The productivity of eucalypt plantations has been advanced in Brazil by deploying genetically advanced planting stock integrated with minimum- or zero tillage, effective weed control and fertilizer application informed by researchbased understanding of stand nutritional requirements (Goncalves et al. 2013). This is somewhat paralleled by longterm improvement of eucalypt productivity in South Africa (Morris 2008; Boreham and Pallett 2009). Second rotation decline in P. radiata in South Australia was reversed and overall regional scale site productivity upgraded, not by any single solution but by implementing a science-based, integrated package of practices operationally (O'Hehir and Nambiar 2010).
This review has focused on what need to be done. It is not within its scope to analyse strategies to bring about these changes in SE Asia, as they will depend heavily on several factors including ownership, investment models in operation and policies. Forestry is a long-term business, and can be viable and successful only if the fundamentals of sustainable production over multiple rotations are understood and managed wisely. It is specially so with short rotation forestry. If productivity fails, the objective of forestry will be lost with it. In simple terms, informed decisions in unison on applications of species selection and genetic improvement, disease and pest management, and soil management especially interrotational practices with due care for the environmental values of the landscape, will together determine whether plantations in SE Asia struggle, survive or succeed.
The Australian Centre for International Agricultural Research (ACIAR) commissioned and funded the review on which this paper is based. Additional funding support was provided by the International Finance Corporation. We thank Dr Philip Smethurst (CSIRO) for discussions and review, and Dr Philip Polglase (CSIRO) and Emeritus Prof. Roger Sands and for their reviews of an earlier draft.
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E.K.S. NAMBIAR (1) and C.E. HARWOOD (2)
(1) CSIRO Sustainable Ecosystems, Black Mountain Laboratories, Canberra, Australia
(2) CSIRO Sustainable Ecosystems, Private Bag 12, Hobart 7001, Australia
TABLE 1 Status of acacia and eucalypt breeding in selected SE Asian countries in 2013 Country Species Area of % of area plantations clonal (ha) China E. dunnii 60 000 0 E. grandis 60 000 80 Eucalyptus 4 M 100 hybrids (3) E. globulus 120 000 0 Indonesia E. pellita 300 000 80 A. mangium 500 000 0 A. crassicarpa 700 000 0 Malaysia A. mangium 250 000 0 Thailand E. camaldulensis 500 000 >90 & hybrids Vietnam E. urophylla 200 000 10 A. auriculiformis 90 000 1 A. mangium 600 000 0 Acacia hybrid 400 000 100 Country Species Size of Generations breeding of breeding (2) population (1) China E. dunnii 300 1 E. grandis >300 2 Eucalyptus >700 (3) 2 hybrids (3) E. globulus 200 1 Indonesia E. pellita 225 2 A. mangium 660 2 A. crassicarpa 300 2 Malaysia A. mangium 200 1-2 Thailand E. camaldulensis 300 2 & hybrids Vietnam E. urophylla 140 2 A. auriculiformis 150 2 A. mangium 120 2 Acacia hybrid 2 Country Species No. of clones deployed operationally China E. dunnii 0 E. grandis 8 Eucalyptus 20 hybrids (3) E. globulus 0 Indonesia E. pellita 20 A. mangium 0 A. crassicarpa 0 Malaysia A. mangium 0 Thailand E. camaldulensis 10 & hybrids Vietnam E. urophylla 6 A. auriculiformis 2 A. mangium 0 Acacia hybrid 12 (1) number of unrelated open-pollinated families from above-average provenances. (2) 1 = first-generation progeny trials established, 2 = second- generation progeny trials established. (3) combinations of E. grandis, E. urophylla, E. camaldulensis, E. tereticornis and E. pellita population size for breeding includes all parental species. TABLE 2 Stem volumes over bark for different genetic treatments in trials comparing improved and unimproved genetic stock of acacias Region and Species Genetic treatments (1) country South Sumatra, A. mangium Local seed source Indonesia (1) Best natural provenance Central and A. auriculiformis Local seed source northern Best natural provenance Vietnam (2) Best seed orchard seed Northern A. mangium Commercial seed source Vietnam (3) A. mangium Best natural provenance A. mangium Seed orchard Acacia hybrid Mix of hybrid clones Region and Species Assessment Wood volume country age (yr) ([m.sup.3]/ha) South Sumatra, A. mangium 5.5 172 Indonesia (1) 250 Central and A. auriculiformis 4 13 northern 19 Vietnam (2) 28 Northern A. mangium 8 73 Vietnam (3) A. mangium 106 A. mangium 124 Acacia hybrid 138 (1) E.B. Hardiyanto, pers.comm. (2013). Volume over bark estimated from height, diameter and form factor. (2) Hai et al. (2008a). Mean of three trials in central and northern Vietnam. Conical tree volumes, calculated from height and dbh. (3) N.D. Kien, pers.comm. (2012). One trial in northernVietnam. Volume over bark estimated from height, diameter and form factor. TABLE 3 Nutrient contents and distribution in selected acacia and eucalypt short-rotation plantations of near-harvest age A. auriculiformis (1) A. mangium (2) Vietnam Indonesia Stand age (yr) 6 6 AGB (t [ha.sup.-1]) Bark 8.0 14.2 Stem 76.6 124.7 AGB 107.5 189.5 N (kg [ha.sup.-1]) Bark 116.5 139.0 Stem 136.5 236.0 AGB 475.2 661.0 P (kg [ha.sup.-1]) Bark 8.1 1.5 Stem 47.7 7.8 AGB 76.8 14.3 K (kg [ha.sup.-1]) Bark 38.7 35.6 Stem 116.4 37.4 AGB 231.5 191.2 Ca (kg [ha.sup.-1]) Bark 24.4 163.7 Stem 15.9 103.5 AGB 67.1 415.8 E. globulus (2) E. grandis (3) Australia S. Africa Stand age (yr) 13 7 AGB (t [ha.sup.-1]) Bark 26.9 12.8 Stem 186.9 107.4 AGB 256.9 133.8 N (kg [ha.sup.-1]) Bark 51.1 42.6 Stem 114.0 77.3 AGB 465.8 249.2 P (kg [ha.sup.-1]) Bark 3.4 6.3 Stem 49.0 4.1 AGB 68.7 17.8 K (kg [ha.sup.-1]) Bark 50.6 52.9 Stem 104.7 81.6 AGB 309.5 189.0 Ca (kg [ha.sup.-1]) Bark 582.9 101.0 Stem 214.9 80.8 AGB 1167.5 248.1 E. tereticornis (4) India Stand age (yr) 7 AGB (t [ha.sup.-1]) Bark 17.9 Stem 46.8 AGB 64.8 N (kg [ha.sup.-1]) Bark 32.5 Stem 74.6 AGB 178.6 P (kg [ha.sup.-1]) Bark 11.0 Stem 20.7 AGB 42.4 K (kg [ha.sup.-1]) Bark 70.7 Stem 90.3 AGB 229.9 Ca (kg [ha.sup.-1]) Bark 170.8 Stem 91.2 AGB 409.1 Source: (1) Vu Dinh Huong, pers.comm.; (2) Yamada et al. (2004); (3) du Toit et al. 2004; (4) Sankaran (1999). TABLE 4 The lowest and highest amounts of wood produced in response to inter-rotation management treatments at a range of second- rotation sites Site Species Stand age (y) Congo: Pointe-Noire E. hybrid 7.0 India: Kerala Punalla E. tereticornis 6.5 Surianelli E. grandis 6.5 Vattavada E. grandis 6.5 Kayampoovam E. tereticornis 6.5 China: Guangdong E. urophylla 7.5 Brazil: Itatinga E. grandis 8.7 South Africa: KZ-Natal E. grandis 5.5 Australia Busselton (WA) E. globulus 10.0 Manjimup (WA) E. globulus 10.0 Indonesia: Sumatra Riau A. mangium 8.0 Sodong A. mangium 7.0 Vietnam: Binh Duong A. auriculiformis 6.0 Site Species Lowest volume ([m.sup.3]/ha) Congo: Pointe-Noire E. hybrid 84.1 India: Kerala Punalla E. tereticornis 78.2 Surianelli E. grandis 166.7 Vattavada E. grandis 328.4 Kayampoovam E. tereticornis 87.4 China: Guangdong E. urophylla 38.9 Brazil: Itatinga E. grandis 176.2 South Africa: KZ-Natal E. grandis 123.0 Australia Busselton (WA) E. globulus 74.0 Manjimup (WA) E. globulus 434.0 Indonesia: Sumatra Riau A. mangium 247.0 Sodong A. mangium 314.0 Vietnam: Binh Duong A. auriculiformis 165.2 Site Species Highest volume Gain (%) ([m.sup.3]/ha) Congo: Pointe-Noire E. hybrid 160.9 91 India: Kerala Punalla E. tereticornis 278.0 256 Surianelli E. grandis 269.5 62 Vattavada E. grandis 350.0 7 Kayampoovam E. tereticornis 140.0 60 China: Guangdong E. urophylla 52.5 35 Brazil: Itatinga E. grandis 276.7 57 South Africa: KZ-Natal E. grandis 146.0 19 Australia Busselton (WA) E. globulus 122.0 65 Manjimup (WA) E. globulus 487.0 12 Indonesia: Sumatra Riau A. mangium 297.6 20 Sodong A. mangium 366.0 18 Vietnam: Binh Duong A. auriculiformis 197.7 20 Source: Revised and updated from Nambiar and Kallio (2008). TABLE 5 Change in soil properties in the surface 10 cm from the end of the first rotation to the harvest of the second rotation. Revised from Tiarks and Ranger (2008) and updated from personal communications from network partners. See Nambiar (2008) for experimental details Site Species Stand age (yr) Congo: Pointe-Noire E. hybrid 7.0 China: Guangdong E. urophylla 7.5 Brazil: Itatinga E. grandis 8.7 South Africa: KZ-Natal E. grandis 5.5 Australia Busselton (WA) E. globulus 10.0 Manjimup (WA) E. globulus 10.0 Indonesia: Sumatra Riau A. mangium 8.0 Sodong A. mangium 7.0 Vietnam: Binh Duong A. auriculiformis 6.0 Site Species SOC (g /kg) Congo: Pointe-Noire E. hybrid [left and right arrow] China: Guangdong E. urophylla [up arrow] Brazil: Itatinga E. grandis [left and right arrow] South Africa: KZ-Natal E. grandis [up arrow][up arrow] Australia Busselton (WA) E. globulus [left and right arrow] Manjimup (WA) E. globulus [left and right arrow] Indonesia: Sumatra Riau A. mangium [up arrow] Sodong A. mangium [left and right arrow] Vietnam: Binh Duong A. auriculiformis [up arrow][up arrow] Site Species Total N (g /kg) Congo: Pointe-Noire E. hybrid [left and right arrow] China: Guangdong E. urophylla [down arrow] Brazil: Itatinga E. grandis [left and right arrow] South Africa: KZ-Natal E. grandis [up arrow] Australia Busselton (WA) E. globulus [left and right arrow] Manjimup (WA) E. globulus [left and right arrow] Indonesia: Sumatra Riau A. mangium [left and right arrow] Sodong A. mangium [left and right arrow] Vietnam: Binh Duong A. auriculiformis [up arrow][up arrow] Site Species Extr. P (mg/kg) Congo: Pointe-Noire E. hybrid NR China: Guangdong E. urophylla [left and right arrow] Brazil: Itatinga E. grandis [down arrow] South Africa: KZ-Natal E. grandis [left and right arrow] Australia Busselton (WA) E. globulus [left and right arrow] Manjimup (WA) E. globulus [left and right arrow] Indonesia: Sumatra Riau A. mangium [left and right arrow] Sodong A. mangium [down arrow] Vietnam: Binh Duong A. auriculiformis [down arrow] Site Species pH Congo: Pointe-Noire E. hybrid NR China: Guangdong E. urophylla [left and right arrow] Brazil: Itatinga E. grandis [left and right arrow] South Africa: KZ-Natal E. grandis [left and right arrow] Australia Busselton (WA) E. globulus [left and right arrow] Manjimup (WA) E. globulus [left and right arrow] Indonesia: Sumatra Riau A. mangium [left and right arrow] Sodong A. mangium [left and right arrow] Vietnam: Binh Duong A. auriculiformis [left and right arrow] Site Species K (c mol/kg) Congo: Pointe-Noire E. hybrid [left and right arrow] China: Guangdong E. urophylla [up arrow] Brazil: Itatinga E. grandis [up arrow] South Africa: KZ-Natal E. grandis [left and right arrow] Australia Busselton (WA) E. globulus [down arrow] Manjimup (WA) E. globulus [down arrow] Indonesia: Sumatra Riau A. mangium [down arrow] Sodong A. mangium [left and right arrow] Vietnam: Binh Duong A. auriculiformis [down arrow] Site Species Ca Congo: Pointe-Noire E. hybrid [down arrow] China: Guangdong E. urophylla [up arrow] Brazil: Itatinga E. grandis [left and right arrow] South Africa: KZ-Natal E. grandis [left and right arrow] Australia Busselton (WA) E. globulus [left and right arrow] Manjimup (WA) E. globulus [left and right arrow] Indonesia: Sumatra Riau A. mangium [down arrow] Sodong A. mangium [left and right arrow] Vietnam: Binh Duong A. auriculiformis [left and right arrow] Site Species Mg Congo: Pointe-Noire E. hybrid [left and right arrow] China: Guangdong E. urophylla [left and right arrow] Brazil: Itatinga E. grandis [up arrow] South Africa: KZ-Natal E. grandis [left and right arrow] Australia Busselton (WA) E. globulus [left and right arrow] Manjimup (WA) E. globulus [left and right arrow] Indonesia: Sumatra Riau A. mangium [down arrow] Sodong A. mangium [left and right arrow] Vietnam: Binh Duong A. auriculiformis [down arrow] [left and right arrow] no change [up arrow] increased [down arrow] decreased [up arrow][up arrow] statistically significant NR not reported. Increases and decreases are only trends and not statistically significant unless specified.
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|Author:||Nambiar, E.K.S.; Harwood, C.E.|
|Publication:||International Forestry Review|
|Date:||Jun 1, 2014|
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